Advance Groundwater Treatment Iron, Manganese, Fluoride and Boron - PowerPoint PPT Presentation

Advance Groundwater Treatment Iron, Manganese, Fluoride and Boron Removal S. K. Sharma, B. Petrusevski, J.C. Schippers October 2001

Groundwater as a Source for Drinking Water Production - 97% of the planet’s freshwater stored in aquifers - groundwater in general of constant & good quality - commonly available close to demand points - relatively low capital & operational costs

Groundwater Use for Drinking Water Production The major source in many countries Region Share of GW (%) people served (%) (million) Asia / Pacific 32 1000-2000 Europe 75 200-500 Latin America 29 150 US 51 135 Australia 15 3 World 1.5-2.0 Billion

Groundwater Use for Drinking Water Production by Country Europe Denmark 100%, Germany 72%, The Netherlands 68% The United Kingdom 27% Asia India (rural) 80%, Philippines 60%, Thailand 50%, Nepal 60%, Bangladesh 90% United States (rural) 96% Many of the largest cities in developing world depends almost completely on groundwater Jakarta, Dhaka, Lima, Mexico City

Problems Associated with the Use of Groundwater - quantities available are limited - the rate of groundwater renewal is very slow (the average recycling time 1,400 years) - contamination by human activities (pesticides, heavy metals, organic micro-pollutant..) - naturally occurring groundwater quality problems (iron, manganese, fluoride, arsenic, boron, methane, ammonium)

Iron � fourth most abundant element on earth crust � a common constituent of groundwater ( 1 to 40 mg/l) No health consequence of iron, taste threshold 0.3 mg/l (WHO, 1996) Problems with iron � Staining, coloration, bad taste � After growth in the distribution system � Incidence of increased turbidity � Increased O&M cost for cleaning pipes

Iron in Groundwater - A Global Problem Developing Countries (In rural areas) - Rejection of hygienically reliable groundwater because of bad taste - People go back to contaminated sources Developed Countries - Higher consumer complaints as iron affects household appliances - Increased cost of cleaning pipes (O&M) - Affects further treatment processes

Iron Removal Methods � Oxidation and Rapid Sand Filtration Oxidation O 2 (Aeration) Cl 2 , KMnO 4 , O 3, H 2 O 2 , ClO 2 � Limestone Filtration � Oxidising Filters (Manganese green sand) � Stabilization (Sequestering ) � Ion Exchange � Sub Surface Removal (Vyredox Method)

Iron Removal Standards WHO 0.3 mg/l Guideline value EC 0.05 mg/l Desired , 0.2 mg/l MAC Dutch Water companies <0.03 mg/l (recommendation). Aeration - Precipitation - Rapid Sand Filtration Most commonly used iron removal method - Simple, Economical, No chemicals

Oxidation of Iron(II) Forms of Iron � Fe (II) - dissolved ( No oxygen) � Fe (III) - insoluble ( Oxygen present) Oxidation Reaction 4Fe 2+ + O 2 + 2H 2 O → 4Fe 3+ + 4OH - 4Fe 3+ + 4OH - + 2(n+1) H 2 O → 2(Fe 2 O 3 .nH 2 O) + 8H + 4Fe 2+ + O 2 + 2(n+2) H 2 O → 2(Fe 2 O 3 .nH 2 O) + 8H + 1 mg of Fe requires 0.14 mg of oxygen

Iron Oxidation Kinetics Stumm & Lee (1961) d [Fe(II)]/dt = - k pO 2 . [OH - ] 2 . [Fe(II)] [Fe(II)] = concentration of Fe(II) (mol/l) t = time (min) = reaction rate constant (l 2 /mol 2 .atm.min) k = 1.0 x10 13 to 8.0 10 13 pO 2 = partial pressure of oxygen (atm) [OH - ] = concentration of hydroxyl ion (mol/l) An increase by one pH unit increases oxidation rate 100 fold Temperature, Alkalinity & Organic matter influence iron oxidation

Sensitivity of Iron Oxidation Kinetics d [Fe(II)]/dt = - k pO 2 . [OH - ] 2 . [Fe(II)] (Stumm & Lee, 1961) 2.5 2.5 2 2 Fe(II) [mg/l] Fe(II) [mg/l] 1.5 1.5 1 1 0.5 0.5 0 0 0 10 20 30 40 50 0 10 20 30 40 50 Time [minutes] Time [minutes] O2 = 1 mg/l O2 = 5 mg/l O2 = 10 mg/l pH = 6.5 pH = 7.0 pH = 7.5 Effect of pH Effect of Oxygen

Example calculation. What is the initial iron oxidation rate, for an oxygen saturated O C) water at pH 7.0 with an initial iron concentration of 5 mg/l. (25 Oxygen partial pressure = f (P - pw) / 101 300 = 0.209(101 300-1 230) / 101 300 = 0.21 atm -5 mol/l Fe(II) = (5 /1000)/56 = 8.9 x 10 + ].[OH - ] = K w = 1.01 x 10 -14 , [H - ) = 1.01 x 10 -14 /1 x 10 -7 = 1.01 x 10 -7 mol/l (OH 13 l 2 /mol 2 .atm.min Assume k = 1.5 x 10 13 . 0.21 . (1.01 x 10 -7 ) 2 . 8.9 x 10 -5 Therefore, d(Fe(II)) /dt = -1.5 x 10 -6 mol/min = -2.86 x 10

What is the percentage of Fe(II) remaining after two and twenty minutes? - ) 2 . t) Fe(II)/Fetot = EXP(-k . p O2 . (OH at t = 2mins -5 . EXP -(1.5x10 13 . 0.21 . (1.01x10 -7 ) 2 . 2) = 8.3 x 10 -5 Fe(II) = 8.9 x 10 or 93% at t = 20 mins, -5 . EXP -(1.5x10 13 . 0.21 . (1.01x10 -7 ) 2 . 20) = 5.7 x Fe(II) = 8.9 x 10 -5 10 or 64%

Reported causes of poor performance of of conventional iron removal process � low oxidation pH � short time for oxidation � negative effect of chlorination � problems related to floc formation � poor selection of effective sand size � iron complexation (by silica and humics) � inappropriate location for regent dosage � deterioration of raw water quality over time

Iron Removal Mechanisms Full understanding of mechanisms involved will help to optimise iron removal process in terms of EFFLUENT QUALITY, PLANT CAPACITY and COSTS Physical/chemical removal � Oxidation and Floc formation � Adsorption Oxidation Mechanism Biological Iron Removal � Iron oxidation mediated by “iron bacteria”

Oxidation and floc formation mechanism Conventional approach � Oxidation of iron(II) to iron(III) � Hydrolysis of iron(III) � Filtration of flocs formed Fe 2+ Fe 3+ Fe(OH) 2+ + Fe(OH) 2 Agglomeration of iron hydroxides Fe(OH) 3 Crystallization Micro flocs - Fe(OH) 4 Filtration

Problems with floc formation mechanism Frequent clogging of filters, shorter filter run • • Incomplete iron oxidation • Colloidal iron passing through the filter More sludge treatment and disposal •

Biological Iron Removal � Oxidation of iron(II) to iron(III) caused by bacteria ( Gallionella, Crenothrix, Sphaerotilus-Lepothrix) � Bacteria derive energy from the oxidation 4Fe 2+ + O 2 + 10 H 2 O = 4 Fe(OH) 3 + 8H + + Q cal. � Optimum pH 6- 8 � Optimum Temperature 10- 15 o C( Gallionella ) , 20 - 25 o C ( Spahaerotilus - Lepothrix) Limitations � Mechanism not fully understood � Temperature and water quality dependent � pH sensitive

Adsorption Oxidation mechanism � No pre oxidation of iron(II) � Removal of iron in iron(II) form � iron(II) adsorption onto filter surface/flocs � oxidation of adsorbed iron(II) and creation of new surface for adsorption Fe 2+ dissolved Fe 2+ adsorbed + O 2 = Fe 3+ newly adsorbed Sand Sand Fe 2+ Sand grain grain grain Fe 2+ adsorbed I II III

Adsorptive Iron Removal - A Conceptual Model Hydrated surface of filter media OH ≡ − ≡ 〈 S OH or S (1) OH Adsorption of iron(II) onto the surface of filter media ≡ S-OH + Fe 2+ � ≡ S-OFe + + H + (2) Oxidation of iron(II) & recreation of adsorption sites OH 1 1 (3) + − + ≡ − + + + → ≡ − 〈 + S OFe 2OH O H S OFe H O 2 2 4 OH 2

Adsorptive Filtration Process Application in water and wastewater treatment increasing � removal to much lower level � works over wider pH range � simultaneous removal of different uncomplexed and complexed metal iron oxide coated sand adsorbs Cu, Pb, Cd, Ni, As � low sludge production

Adsorptive iron removal � is the natural process : occurring in the filters of iron removal plant and in sub surface iron removal � increased efficiency of the filters after the development of coating well known � Sometimes referred to as “ Catalytic Iron Removal ” � appropriate for anoxic groundwater

Previous research at IHE on iron removal Adsorption Oxidation mechanism gives • Lower head loss, longer filter run • Higher removal efficiency • Shorter ripening time In addition, • No/less problem of sludge • Reduction in frequency of backwash

Factors affecting iron removal mechanisms A. Water quality parameters � pH � Oxygen concentration � Alkalinity � 2- ) Ionic concentration (Mn, Ca, SO 4 B. Process conditions � Pre oxidation time - depth of supernatant � Type and size of the filter media � Age of the filter media - characteristics of the coating

Iron(II) Adsorption Isotherms pH = 7.0, temp = 20 o C 1 Coated sand Iron(II) adsorbed (g/m2) 0.1 0.01 New sand 0.001 1 10 Equilibrium concentration Ce (mg/l)

Adsorption Capacities of Different Filter Media K = iron(II) adsorbed per unit surface area (Q mg/m 2 ) at iron(II) equilibrium concentration C e = 1 mg/l 20 Isotherm Constant, K (mg/m2) pH = 6.5 15 10 5 0 Anthracite Basalt Pumice Sand Magnetite Olivine Limestone

Head Loss with Different Mechanisms Filtration rate = 10 m/h, Depth of supernatant = 1 m Sand size = 0.5 - 0.8 mm, Influent iron = 1.8 mg/l Floc Filtration Adsorptive Filtration 3 Bed depth = 1.8 m Head loss (m) 2.5 2 1.5 1 0.5 0 20 40 60 80 Run time (hours)

Manganese in Groundwater Mainly present in GW as Mn 2+ (dissolved) Frequently coexists with Fe 2+ - causes similar problems - taste and staining problems more severe Standards WHO 0.1 mg/l Guideline value EC 0.02 mg/l Desired, 0.05 mg/l MAC